Elastomeric polyurethane foams for applications, such as for example, shoe soles, desirably exhibit good physical properties including abrasion resistance, flexibility and durability. Typically, these foams are prepared by reacting an organic isocyanate with a substance having at least one isocyanate reactive group in the presence of a catalyst, blowing agent and a variety of optional additives. The reaction is generally carried out in a mold whereby a higher density skin forms at the interface of the reaction mixture and the inner surface of the mold.
At the present time, one of the most common types of blowing agents utilized in the preparation of such polyurethane foams are the chlorofluorocarbons (CFCs), e.g., freon-11. Shoe soles comprised of these polyurethane foams, especially freon blown polyurethane foams, have a very thick skin, are abrasion resistant, stable and exhibit excellent flex fatigue properties.
However, since industry is faced with a mandate to reduce and eventually eliminate the use of CFCs due to environmental concerns, it is necessary to find an alternative blowing agent. Water is a highly desirable blowing agent and has been used as a blowing agent to prepare lower density polyurethane foams. However, heretofore, water has been found to be generally unacceptable as the sole blowing agent, especially in the preparation of foams intended for use as shoe soles. The density of the polyurethane foams prepared utilizing water as the sole blowing agent is generally too low to provide the adequate stability and cushioning demanded for modern shoes. In addition, shoe soles comprised of water blown polyurethane foams do not have thick skin and exhibit very poor flex fatigue properties. Thus, the shoe soles crack very easily after several flexing operations.
The present inventor, however, has successfully found a reaction system for preparing an all water-blown microcellular integral skin polyurethane foam and has thus overcome the problems described hereinabove. More specifically, utilizing the reaction system of the present invention, the polyurethane foam prepared therefrom has improved flex fatigue properties, making it an ideal material for use in shoe sole compositions. The inventor has found that improved flex properties are realized if the isocyanate reaction component contains a mixture of two high ethylene oxide containing polyols.